This application is based upon and claims the benefit of Japanese Patent Applications No. 2000-46793 filed on Feb. 18, 2000, and No. 2000-327504 filed on Oct. 26, 2000, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an angular velocity sensor having a weight portion connected to a basal portion via a beam portion so as to detect angular velocity based on the oscillation of the weight portion. The sensor is applicable to an angular velocity sensor unit such as a vehicular control system, a vehicular turn-over detecting system, a navigation system and a blur preventing system of optical devices which need to sense angular velocity.
2. Description of the Related Art
Oscillation type angular velocity sensors formed by processing a semiconductor substrate (SOI substrate and the like) by using micro-machining technology for the purpose of miniaturization and cost reduction have been reported lately. For instance, Japanese Patent Application Laid-Open Nos. Hei. 9-119942, Hei. 6-123632, Hei. 8-220125, Hei. 11-248733 and other have proposed such angular velocity sensors.
These angular velocity sensors have a weight portion (oscillator) that is excited and oscillated in a first direction (driving direction, x-axis direction). When the weight portion is turned about an angular velocity axis (z-axis), Corioli""s force is generated at the weight portion in a second direction (detecting direction, y-axis direction) that crosses at right angles with the first direction. This Corioli""s force is transmitted to a detecting element having movable and stationary electrodes by a beam portion, i.e., by an oscillation spring. In the detecting element, a capacity between the movable electrode and the stationary electrode changes due to the displacement of the movable electrode, thus detecting an output value of the angular velocity.
When it is supposed that the sensor could be fabricated conforming to its design without any processing error in the beam structure composing the angular velocity sensor, the sensor will operate accurately as described above. However, when the beam portion as the oscillation spring, the driving electrode and others have processing errors (e.g., the thickness of the beam portion is erroneous), the oscillation may leak in the y-axis direction, i.e., in the detecting direction, for instance even if the weight portion should be oscillated only in the x-axis direction during the driving oscillation thereof. In such a case, the capacity of the detecting portion changes even if the angular velocity is zero, causing an error in the output value of the angular velocity.
Accordingly, it has been required to process the prior art sensors as accurately as possible in order to prevent the leak of driving oscillation of the weight portion in the detecting direction (hereinafter referred to as unnecessary oscillation) and the processing precision has determined the performance of the sensor. However, the processing error is inevitable in the angular velocity sensor that is formed by processing the semiconductor substrate, and there is a limit in the reduction of the unnecessary oscillation.
The present invention has been made in view of the above problems. An object of the present invention is, in an oscillation type angular velocity sensor having a weight portion, to reduce an error in output value of angular velocity by eliminating leak (unnecessary oscillation) of oscillation of the weight portion in a detecting direction caused by a processing error.
According to the present invention, briefly, an angular velocity sensor has a weight portion that can be drive-oscillated in a first direction, and can be oscillated in a second direction when an angular velocity is applied around an angular velocity axis, vertical to the first and second directions, in state where the weight portion is drive-oscillated. The angular velocity sensor further has unnecessary oscillation suppressing means for applying an external force to the weight portion in the second direction to prevent the weight portion from being drive-oscillated in a direction other than the first direction.
Thus, in the angular velocity sensor of the invention, the unnecessary oscillation suppressing means applies the external force to the weigh portion in the second direction, and cancels an oscillation component in the second direction that causes unnecessary oscillation in the driving oscillation. As a result, the weight portion can be drive-oscillated only in the first direction even when a beam portion has a processing error. No leak of oscillation of the weight portion occurs in the second (detecting) direction, and an error in output value of the sensor can be lowered.